The present invention relates generally to equipment for treating molten metals. More particularly, the invention relates to an injection lance for fluxing molten aluminum alloys.
Molten aluminum alloys, unless treated, tend to contain in solution a certain amount of undesirable impurities, such as oxides and/or gases. It is thus desirable to treat molten aluminum alloys to reduce the amounts of undesirable impurities. In general, fluxes can be added to molten aluminum alloys to chemically and mechanically react with undesirable impurities and thereby facilitate removal of the impurities. The addition of fluxes to aluminum alloy melts is known in the art and is described generally in Metals Handbook (8th Ed.), Vol. 5, p 395-397, which is hereby expressly incorporated by reference.
One popular technique for introducing flux into a molten aluminum alloy is to sprinkle flux onto the surface of the molten metal. The flux is then folded into the melt by an operator using an instrument such as a stirring paddle. This method, however, is limited in that it requires large amounts of physical labor for stirring. Further, in many instances, greater amounts of flux than normal may be needed to overcome problems associated with the stirring action, such as an increased introduction of undesirable gases into the melt.
Another popular technique for introducing flux into an aluminum alloy melt is to suspend a powdered flux in a carrier gas and to project the flux-gas suspension into the molten aluminum alloy using a conduit or lance. Typically, a lance employed in this technique has an open discharge orifice which is placed beneath the surface of the molten aluminum alloy. Further, the lance is equipped so that the gas-flux mixture is introduced into the melt under pressure. The combination of gas pressure and flux composition of the flux permits a chemical-mechanical reaction to occur between the impurities and the flux. The chemical-mechanical reaction, in turn, generally results in causing undesirable impurities to rise to the surface of the molten aluminum alloy where they can be removed by methods such as skimming methods.
A troublesome problem with the latter flux injection technology has been that inside portions of the lance tend to become clogged or plugged during normal use, thereby requiring frequent and time-consuming cleaning. In some instances, the plugging becomes so severe that the lance requires complete replacement. It is not uncommon for a flux injection lance to become clogged to an unworkable level in only a four to ten minute time frame. Thus, after approximately every four to ten minutes of lance operation, an operator must stop the injection process, wait for it to cool to a suitable handling temperature and clean the lance. In severe cases, the operator must further replace the clogged lance with a new one.
I have identified the cause of this problem as being largely due to premature melting of the flux material before being discharged from the lance. In many instances where the flux material is particulate matter, the outer surfaces of the flux particles melt and fuse with nearby particles to form an aggregation of flux particles. The aggregation of flux particles then tends to adhere to inside walls of the lance. After a period of time, the aggregation of flux particles grows so as to cause a flux buildup on the lance walls. The flux buildup, in turn, tends to constrict passages of the lance, such as the discharge orifice and thereby restricts further passage of flux therethrough.
The fluxes popularly used in molten aluminum alloy treatment typically include admixtures of various materials, including suitable salts, which generally have a melting point significantly lower than that of aluminum. For instance, many suitable flux salts typically have a melting point of roughly 300.degree.-800.degree. Fahrenheit or less. The molten aluminum alloy being treated with such salts is typically maintained at a temperature of about 1300.degree. Fahrenheit or higher. In the flux injection process, it is desired to project the suspended flux particles through the lance and into the molten aluminum, via the discharge orifice, before the flux salt particles have a chance to heat up and melt. Thus, it is preferable to prevent the flux from melting until after it is discharged from the lance and is within the molten aluminum alloy.
Using conventional lances, this is frequently a problem. Conventional lances used in molten aluminum alloy treatment typically comprise a cylinder of graphite or silicon carbide with a thinwall steel tubing insert through which the flux is conveyed. During the course of treatment, the cylinder heats up by direct contact with the molten metal. This heat is then transferred directly to the thinwall steel tubing and causes the temperature within the tubing to rise above the melting point of the flux. As a consequence, the flux that is suspended in the tubing melts, and then adheres to the inside wall of the cylinder as described above. Thus, flux buildup occurs, and often times completely plugs up the cylinder, so that time consuming and expensive cleaning is required.
The present invention overcomes various problems found in prior art aluminum alloy treatment flux injection methods by providing an improved and relatively easy-to-operate lance having a discharge orifice in which a flux injector conduit is provided with a gaseous coolant circulation system which envelops at least a portion of the conduit. The gaseous coolant circulation system permits air, or other gaseous heat removal medium, to thermally contact the conduit, defining a heat removal site about the conduit. At least a portion of the heat removal site is thermally insulated from the molten aluminum, resulting in the temperature at the discharge orifice, and along the conduit, remaining below the fusion point or melting point of the flux. In this way, melting and adhesion of the flux to the conduit along the conduit and at the discharge orifice is minimized.
In accordance with the inventive method, a powdered flux is mixed with a moving or pressurized injection carrier gas to form an injection gas-flux suspension. The lance is placed in the molten aluminum with the discharge orifice submersed in the aluminum. The injection gas-flux suspension is flowed through the lance and into the molten aluminum through the discharge orifice and the lance is cooled along the conduit and at its discharge end by passing a cooling gas adjacent to the conduit and discharge orifice. The cooling gas is provided to a chamber on the lance adjacent the conduit and discharge orifice, which chamber is sealed from the molten aluminum. In addition, heat transfer is minimized between the chamber and the molten aluminum by insulating the exterior of the chamber. In this fashion, the temperature along the conduit and at the discharge orifice is maintained below the melting point of the flux to thereby minimize adhesion of the flux to the inside walls of the conduit.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.